1. Field of the Invention
The present invention relates to measurement of light.
2. Description of the Prior Art
There has been conventionally known a method for measuring a device under test (DUT) by a terahertz detector receiving terahertz light A (in a form of pulse) which has been fed from a terahertz emitter to the DUT, and has transmitted through the DUT, and light B having the pulse period which is slightly different from the pulse period of terahertz light A (refer to ABSTRACT of Patent Document 1, for example).
According to the above-described conventional technology, a detection result by the terahertz detector, and a trigger signal indicating an origin of time are fed to a digital oscilloscope to measure the DUT. It should be noted that the trigger signal is generated by obtaining sum frequency generation (SFG) cross-correlation of a part of an optical pulse (a probe light fed to the terahertz detector) output from a first femtosecond laser and a part of an optical pulse (a pump light fed to the terahertz emitter) output from a second femtosecond laser (refer to FIG. 20 of Patent Document 1, for example).
It should be noted that the trigger signal is also described in Non-Patent Documents 1 to 7.    (Patent Document 1) WO 2006/092874, Pamphlet    (Non-Patent Document 1) Bartels et al, “Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling”, Rev. Sci. Instrum., vol. 78, pp. 035107 (2007)    (Non-Patent Document 2) T. Yasui et al, “Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition”, Appl. Phys. Lett., vol. 87, pp. 061101 (2005)    (Non-Patent Document 3) A. Bartels et al, “High-resolution THz spectrometer with kHz scan rates”, Optics express, vol. 14, pp. 430 (2006)    (Non-Patent Document 4) A. Bartels et al, “Femtosecond time-resolved optical pump-probe spectroscopy at kilohertz-scan-rates over nanosecond-time-delays without mechanical delay line”, Appl. Phys. Lett., vol. 88, pp. 041117 (2006)    (Non-Patent Document 5) C. Janke et al, “Asynchronous optical sampling for high-speed characterization of integrated resonant terahertz sensors”, Optics Letters, vol. 30, pp. 1405 (2005)    (Non-Patent Document 6) Y. Takagi et al, “Subpicosecond optical sampling spectrometer using asynchronous tunable mode-locked lasers”, Rev. Sci. Instrum., vol. 70, pp. 2218 (1999)    (Non-Patent Document 7) IEICE Technical Report, OCS98-53